This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-051476, filed Feb. 26, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a cathode ray tube apparatus and, more particularly, to a color cathode ray tube apparatus which reduces the elliptic distortion of an electron beam spot shape at the periphery of a phosphor screen and stably provides a good image quality.
In general, a color cathode ray tube apparatus comprises an in-line type electron gun assembly for emitting three electron beams horizontally in a line, i.e., a center beam and a pair of side beams that pass through the same horizontal plane, and a deflection yoke for generating a nonuniform deflection magnetic field for deflecting the three electron beams horizontally and vertically. This nonuniform deflection magnetic field is formed from a pincushion type horizontal deflection magnetic field and barrel type vertical deflection magnetic field. Three electron beams emitted by the electron gun assembly are focused on corresponding phosphor layers on the phosphor screen by a nonuniform deflection magnetic field generated by the deflection yoke while they undergo self-convergence as they travel toward the phosphor screen. Then, a color image is displayed on the phosphor screen.
As the electron gun assembly for emitting three electron beams, an electron gun assembly of a QPF (Quadru-Potential Focus) dynamic astigmatism correction and focus type comprises an array of three cathodes K, and first to sixth grids G1 to G6 which are sequentially laid out toward the phosphor screen and integrally supported, as shown in FIG. 4. Each of the grids G1 to G6 has three electron beam apertures corresponding to the three aligned cathodes K.
In this electron gun assembly, each cathode K receives a voltage of about 150 V, and the first grid G1 is grounded. The second grid G2 is connected to the fourth grid G4 in the tube, and receives a voltage of about 700 V. The third grid G3 is connected to a (5-1)th grid G5-1 in the tube, and receives a voltage of about 6 kV. A (5-2)th grid G5-2 receives a voltage of about 6 kV. The sixth grid G6 receives a high voltage of about 26 kV.
These voltages are applied to the cathodes and grids to form an electron beam generator 8, pre-focusing lens 9, UPF (Uni-Potential Focus) type sub-lens 10, and BPF (Bi-Potential Focus) type main lens 11.
The electron beam generator 8 is made up of the cathodes K, and first and second grids G1 and G2, generates an electron beam, and forms an object point with respect to the main lens 11. The pre-focusing lens 9 is made up of the second and third grids G2 and G3, and preliminarily focuses the electron beam emitted by the triode 8. The sub-lens 10 is made up of the third, fourth, and (5-1)th grids G3, G4, and G5-1, and further preliminarily focuses the electron beam which was preliminarily focused by the pre-focusing lens 9. The main lens 11 is made up of the (5-2)th and sixth grids G5-2 and G6, and finally focuses the preliminarily focused electron beam on the phosphor screen. Note that a lens including the sub-lens 10 and main lens 11 will be called a main lens system 13.
In deflecting an electron beam to the periphery of the phosphor screen by a nonuniform magnetic field generated by the deflection yoke, the (5-2)th grid G5-2 receives a voltage set in advance in accordance with the deflection distance. This voltage parabolically changes depending on the electron beam deflection amount such that the voltage minimizes when the electron beam is focused on the center of the phosphor screen, and maximizes when the electron beam is deflected and focused on the corners of the phosphor screen.
When the electron beam is deflected to a corner of the phosphor screen, the potential difference between the (5-2)th and sixth grids G5-2 and G6 becomes smallest, and the lens power of the main lens 11 becomes weakest. At the same time, the (5-1)th and (5-2)th grids G5-1 and G5-2 form a potential difference to form a quadrupole lens 12. The quadrupole lens 12 formed at this time has the highest lens power because of the largest potential difference between the grids G5-1 and G5-2. The quadrupole lens 12 is set to achieve horizontal focusing action and vertical divergent action.
As the electron beam is deflected to increase the distance from the electron gun assembly to the phosphor screen, and set the object point apart, the main lens power weakens. At the same time, a quadrupole lens 12 for compensating for a deflection error caused by the horizontal and vertical deflection magnetic fields of the deflection yoke is generated. The lens power of the quadrupole lens 12 increases depending on the deflection amount.
To improve the image quality of the color cathode ray tube apparatus, the focusing characteristic on the phosphor screen must be improved. Particularly in a color cathode ray tube which incorporates an electron gun assembly for emitting three electron beams in a line, the beam spot on the phosphor screen generates an elliptic distortion core 1 and blur 2 owing to the deflection error, as shown in FIG. 5A.
In general, as shown in FIG. 5B, the blur 2 can be prevented according to the deflection error compensation method by constituting a low-voltage electrode forming a main lens by a plurality of grids such as the (5-1)th and (5-2)th grids G5-1 and G5-2, and generating a quadrupole lens in accordance with deflection of the electron beam between these grids, like a dynamic astigmatism correction and focus type electron gun assembly. However, as shown in FIG. 5B, the elliptic distortion of a horizontally expanded beam spot still remains at the ends of the horizontal and diagonal axes of the phosphor screen. This elliptic distortion generates moire or the like due to interference with a shadow mask, which makes it difficult to see, e.g., a character formed by an electron beam spot.
The elliptic distortion of the beam spot will be explained using an optical lens model.
FIG. 6A shows a lens model in a no-deflection state in which an electron beam is focused on the center of the phosphor screen without any deflection. FIG. 6B shows a lens model in a deflection state in which an electron beam is deflected and focused on the periphery of the phosphor screen.
The beam spot size on a phosphor screen SCN depends on a magnification M. Let Mh be the horizontal magnification of the electron beam, and Mv be the vertical magnification. Then, M can be given by
M=Divergent Angle xcex1o/Incident Angle xcex1i
That is,
Mh (Horizontal Magnification)=xcex1oh (Horizontal Divergent Angle)/xcex1ih (Horizontal Incident Angle)
Mv (Vertical Magnification)=xcex1ov (Vertical Divergent Angle)/xcex1iv (Vertical Incident Angle)
For xcex1oh=xcex1ov, when the electron beam is not deflected as shown in FIG. 6A, the electron beam is influenced by almost the same focusing action both in horizontal and vertical directions H and V by the sub-lens 10 and main lens 11. This yields xcex1ih=xcex1iv and Mh=Mv.
When the electron beam is deflected as shown in FIG. 6B, the quadrupole lens 12 having divergent action in the vertical direction V and focusing action in the horizontal direction H is formed between the sub-lens 10 and main lens 11 so as to compensate for the influence of a deflection error 14 having focusing action in the vertical direction V and divergent action in the horizontal direction H. This yields xcex1ih less than xcex1iv and Mv less than Mh.
As a result, the beam spot shape of the electron beam becomes circular at the center of the phosphor screen, but is horizontally elongated at the periphery of the phosphor screen.
To prevent this, an electron gun assembly having a so-called double quadrupole structure is proposed. As shown in FIG. 12, this electron gun assembly has almost the same structure as that shown in FIG. 4 except that the third grid G3 is made up of (3-1)th and (3-2)th grids G3-1 and G3-2. The (3-2)th grid G3-2 is connected to a (5-2)th grid G5-2, and receives a parabolic voltage when the electron beam is deflected. This applied voltage forms a quadrupole lens 14 which dynamically changes in synchronism with the deflection magnetic field, between the (3-1)th and (3-2)th grids G3-1 and G3-2 when the electron beam is deflected.
The double quadrupole type electron gun assembly will be explained using an optical lens model.
FIG. 7A shows a lens model when the electron beam is not deflected, and FIG. 7B shows a lens model when the electron beam is deflected.
For xcex1oh=xcex1ov, when the electron beam is not deflected as shown in FIG. 7A, the electron beam is influenced by almost the same focusing action both in the horizontal and vertical directions H and V by the sub-lens 10 and main lens 11. This results in xcex1ih=xcex1iv and Mh=Mv. Hence, a circular beam spot can be formed at the center of the phosphor screen SCN.
When the electron beam is deflected as shown in FIG. 7B, a first quadrupole lens 12A is formed on the cathode side of the sub-lens 10, and a second quadrupole lens 12B is formed between the sub-lens 10 and main lens 11. The first quadrupole lens 12A has focusing action in the vertical direction V and divergent action in the horizontal direction H. The second quadrupole lens 12B has divergent action in the vertical direction V and focusing action in the horizontal direction H.
This results in xcex1ih=xcex1iv and Mh=Mv. In the theory of magnification, a circular beam spot can be formed even at the periphery of the phosphor screen.
However, in this double quadrupole type electron gun assembly, a bundle of electron beams in passing through the main lens 11 have a large horizontal diameter, and are readily influenced by the spherical aberration of the main lens 11. To efficiently operate the double quadrupole lenses, the strong horizontal divergent action and vertical focusing action of the first quadrupole lens 12A and the strong horizontal focusing action and vertical divergent action of the second quadrupole lens 12B must be combined. However, the combination of two quadrupole lenses having strong lens powers increases the spherical aberration of the main lens to inhibit the beam spot shape from becoming circular at the periphery of the phosphor screen SCN.
To reduce the spherical aberration of the main lens caused by the double quadrupole type electron gun assembly, the divergent angle xcex1o is effectively set sufficiently small. When the divergent angle xcex1o is small, the diameter of the virtual object point of the electron beam generally increases. Even if the double quadrupole type electron gun assembly can form a circular electron beam spot on the entire phosphor screen SCN, the beam spot enlarges, resulting in poor image quality.
To prevent the beam spot from enlarging, it is effective to set a small magnification M for the main lens system made up of the sub-lens and main lens. In the QPF type electron gun assembly constituting the main lens system, a main lens system having a large magnification M is formed by giving the sub-lens 10 a strong focusing power and the main lens 11 a weak focusing power, as shown in FIG. 8A. Moreover, a main lens system having a small magnification M is formed by giving the sub-lens 10 a weak focusing power and the main lens 11 a strong focusing power, as shown in FIG. 8B.
By changing the balance between the sub-lens and main lens, the magnification M of the lens system can be decreased relatively easily.
More specifically, a circular beam spot can be formed on the entire phosphor screen by operating the main lens system having a small magnification M, the electron beam having a small divergent angle, and the double quadrupole lenses made of strong quadrupole lenses.
However, in the main lens system having a small magnification, the beam spot diameter, i.e., spot size on the phosphor screen readily changes upon a change in focusing voltage Vf applied to the (3-2)th and (5-2)th grids G3-2 and G5-2. In the main lens system having a small magnification, as shown in FIG. 8B, the beam spot diameter steeply changes with respect to the focusing voltage Vf. This phenomenon appears in an image as degradation of the focusing characteristic when the parabolic voltage applied to the (5-2)th and (3-2)th grids G5-2 and G3-2 deviates from a predetermined voltage. Thus, an accurate external application voltage must be applied, which makes it difficult to design the driver of such color cathode ray tube apparatus, and increases its cost.
As described above, to improve the image quality of the color cathode ray tube apparatus, it is necessary to maintain a good focusing state on the entire phosphor screen and suppress the elliptic distortion of the electronic beam spot. In the conventional QPF dynamic astigmatism correction and focus type electron gun assembly, a proper parabolic voltage is applied to the low-voltage electrode of the main lens to change the lens power of the main lens and form a quadrupole lens which dynamically changes. This prevents generation of the vertical blur of an electron beam caused by the deflection error.
However, the elliptic distortion of the beam spot at the periphery of the phosphor screen is conspicuous. The double quadrupole method adopted to reduce the elliptic distortion of the beam spot can form a circular beam spot on the entire screen. To efficiently operate the double quadrupole lenses, an electron beam having a small divergent angle must land on the phosphor screen. Further, a main lens system having a small magnification M must be constituted.
In an electron gun assembly having the main lens system with a small magnification M, the beam spot diameter on the phosphor screen greatly changes upon a change in focusing voltage Vf. When the parabolic voltage applied outside the color cathode ray tube deviates from a predetermined voltage, the image degrades remarkably. A circuit for driving the color cathode ray tube is difficult to design, and its cost increases.
The present invention has been made to overcome the conventional drawbacks, and has as its object to provide a cathode ray tube apparatus having stable performance of suppressing the elliptic distortion of a beam spot on the entire phosphor screen and obtaining a good focusing characteristic on the entire phosphor screen.
To achieve the above objects, according to the present invention, there is provided a cathode ray tube apparatus comprising an electron gun assembly which has a cathode and a plurality of grid electrodes sequentially laid out from the cathode toward a phosphor screen, and emits an electron beam, and a deflection device for forming a deflection magnetic field for horizontally and vertically deflecting the electron beam emitted by the electron gun assembly,
the electron gun assembly including
an electron beam generator for generating an electron beam,
a pre-focusing lens for preliminarily focusing the electron beam emitted by the electron beam generator,
a sub-lens which has lens action with a weaker horizontal focusing power than a vertical focusing power, and further preliminarily focuses the electron beam which was preliminarily focused by the pre-focusing lens,
a main lens which has lens action with a stronger horizontal focusing power than a vertical focusing power, and focuses the electron beam preliminarily focused by the sub-lens on the phosphor screen, and
voltage application means for applying to each grid electrode of the electron gun assembly a voltage which forms first and second multipole lenses between the pre-focusing lens and the main lens when the electron beam emitted by the electron gun assembly is not deflected and is focused on a center of the phosphor screen, and weakens lens actions of the first and second multipole lenses with an increase in electron beam deflection amount when the electron beam is deflected.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.